1. Field of the Invention
The present invention relates to a method for measuring an electron energy distribution in a plasma region in a film forming apparatus or a plasma etching apparatus, in which plasma is generated by using a high frequency power, thereby to form film. The invention relates to an apparatus for measuring the same.
2. Description of the Related Art
Generally, in a plasma processing apparatus utilizing plasma generated by high frequency power application to perform various film formings, which is represented by the plasma CVD, the plasma sputtering the plasma etching, it is essential that plasma be generated stably and with excellent reproducibility. Therefore, it is necessary to measure electron energy distribution in plasma to control generation of the plasma on the basis of the measured results. Particularly, since the plasma is generated by high frequency power in almost all cases, it is important to perform measurement of the electron energy distribution in the high frequency plasma accurately.
Conventionally, such a method is widely employed that, by utilizing the fact that the relationship between an electron current collected from a probe or a heating probe inserted in the plasma region and the electron temperature of a plasma (one of the bias voltage and the plasma energy of the probe,) is a constant the plasma electron temperature is measured.
In such a method, however, there is such a drawback that it is necessary to arrange a complicated protection circuit in order to remove the influence of high frequency potential vibration which always exists in the plasma region on the probe. In a method for determining an electron temperature using such a probe, since a voltage-current characteristic of a probe which has a non-linear characteristic, and the plasma potential vibrates due to the high frequency vibration of a power supply voltage, a settling current generated by this vibration is captured in the probe simultaneously with the electron current. Therefore, the current captured by the probe is different from the electron current reflecting the accurate electron temperature, so that control on plasma generation is made impossible in principle.
As a measure to solve this drawback, a new high frequency current removing circuit is added to this probe circuit and the settling current is cancelled by applying to the probe a high frequency voltage having the same magnitude as the vibration voltage in the plasma and an inverse phase to the phase of the vibration voltage.
In this method, however, since the voltage value with the inverse phase to be applied to the probe depends on a capacitance of a sheath existing between the probe and the plasma, it is difficult to obtain the electron current and it is difficult to apply a voltage having the inverse phase and exactly the same value as the plasma potential to the probe circuit.
In view of the above, the present inventor has proposed in a Jpn. Pat. Appln. KOKAI No. 2-30098 publication a technique where a probe is maintained in a state that it can emit thermal electrons, a half wave voltage of an alternative current is applied to the probe, a floating potential difference between a no-voltage period and a voltage period is obtained so that a plasma electron temperature is measured on the basis of the difference. However, in this technique, there is included such a problem that an electron temperature corresponding to an average energy in Maxwell distribution can only be measured and a general electron energy distribution can not be measured.